The present invention relates to processes for preparing high-purity phosphorus pentachloride in the form of a freely-flowing crystalline powder.
Numerous processes are known for preparing phosphorus pentachloride, PCl.sub.5. The most usual of these involves reacting chlorine with phosphorus trichloride, PCl.sub.3, either in the dry form or in a solvent.
The dry method is the oldest and was described in BIOS Final Report No. 562. This was the batch process in which liquid phosphorus trichloride was introduced in the form of slender streams at the top of a steel tower containing a rising stream of gaseous chlorine. The solid phosphorus pentachloride was recovered at the bottom of the tower.
An analogous process conducted on a continuous basis has been described by Van Wazer in the Kirk-Othmer, Encyclopedia of Chemical Technology (2d Ed.) 15, 307. U.S. Pat. No. 1,914,750 describes the reaction of liquid chlorine with phosphorus trichloride at a temperature and a pressure such that the phosphorus pentachloride does not sublime. An apparatus for the chlorination of phosphorus trichloride in a closed vessel with automatic control and safety devices is described in Ind. Chemist. Chem. Manufact. 25, 517-20 (1949). Other references are cited in Gmelins Handbuch der Anorganischen Chemie, under the heading "Phosphorus", Vol. C, page 435.
The dry techniques present numerous disadvantages which are, on the one hand, technical problems principally arising from the difficulty of dissipating the heat of reaction which often causes local overheating harmful to normal discharge of the reaction product (because the phosphorus pentachloride sets up into a mass in the reactor) and to the efficient utilization of material. On the other hand, these processes provide an inferior quality phosphorus pentachloride, such product having in effect undesirable coarseness and comprising irregular pieces which must be broken up. Further, this material contains in an occluded form significant quantities of phosphorus trichloride which has not reacted. Finally, these materials retain all of the impurities found in the raw materials; thus, iron and arsenic frequently occur in the product, as do corrosion products.
Accordingly, this dry technique is increasingly being abandoned these days in favor of the so called "solvent" technique. In this latter technique, the phosphorus trichloride is treated with gaseous chlorine in the midst of an inert solvent contained in an agitated reactor equipped with a cooling jacket and a reflux condenser.
After complete reaction of the phosphorus trichloride and cooling, the suspension of phosphorus pentachloride crystals so obtained is then filtered and dried. The greatest part of the impurities remains in the solvent which is recycled as-is or after distillation. The solvent most conveniently utilized is carbon tetrachloride, and attention is called to the Van Wazer reference in this regard. Other possible solvents include tetrachloroethane, chlorobenzene or even phosphoryl chloride, POCl.sub.3 (U.S. Pat. No. 1,906,440).